As noted in Part 1 on Ancaster stone and it’s quarries, this Lincolnshire limestone, while used occasionally in London in Roman and Mediaeval periods, became more regularly used in the late 19thC as ashlar building and facing blocks. https://buildinglondon.blog/2022/08/06/44-ancaster-and-its-streaky-bacon-stone-pt1/
It was used for the glorious Midland Grand Hotel at St Pancras Station in 1873    to re-face the 17thC Lincoln’s Inn Chapel in the 1880s  to face Holborn Library and Town Hall in 1894 and 1908   the Hackney Central Library in 1908   and to re-face the Clapton Round Chapel, which this post concentrates on, in 1906.
Clapton Park Round Chapel
Clapton Round Chapel  was built in 1869-71 for the congregation of the Hackney Congregationalist church which had been established in 1804. They had been meeting in the famous Old Gravel Pit Chapel,  where radical non-conformists Richard Price of Newington Green Chapel  and the scientist Joseph Priestley  had controversially and sensationally preached to their Unitarian church, which had moved just down the road to the New Gravel Pit Chapel in 1809, itself demolished in 1969. Both Price and Priestley were involved with the important and infamous radical ‘dissenting’, New College, Hackney, the so -called “ the slaughterhouse of Christianity”  only a few 100 yards from where the Round Chapel would later be built. The shell of the Old Gravel Pit Chapel surprisingly still exists as part of a clothes shop. Some of the cast iron pillars that would have held the balconies are evident and there is a Blue Plaque down a side alley. 
By the 1860s this congregation had expanded significantly and a new chapel was needed and built, called the Clapton Park Chapel, for the large new middle class housing area just built to the east, and opened in 1871.
The Historic England listing states of the building “1869-71 by H Fuller. Large 2-storey building of ashlar stonework in Italian style with round-arched windows and much arcading. Wide, rounded west end with projecting ground floor and balustraded parapet. 4-stage octagonal towers, with conical slated roofs, flank the east end: and 2 similar smaller towers flank the western curve. 4 flights of steps radiate from curve and towers. 4-bay returns have full height arcading. Raised attic with round windows. Interior with continuous gallery to 3 sides supported on cast iron columns, which also support an upper tier of taller columns rising to a continuous cast iron arcade with openwork cast iron spandrels. Round headed archway with Corinthian columns to east end. Contemporary pulpit with double flight of stairs, organ and organ case to east end and timber pews. The .. [chapel] … and the Sunday School form a group with No 1A Glenarm Road.” 
The chapel was built with what is described as “Redhill stone” in ‘Hackney: Protestant Nonconformity’, in A History of the County of Middlesex: 
That’s a bit odd though as doesn’t appear to be anything called ‘Redhill Stone’ that fits the bill. There are references online to ‘Redhill Stone’ in Shropshire and a roadstone quarry in Leicestershire around this time but it almost certainly refers to Redhill in Surrey. Peter …. From the Wealden Caving group suggests this could be the hard Greensand cap taken from the Fullers Earth Quarry that was operating at that time, but looking at the stone, it seems maybe more likely that it is mis-named Reigate stone or Merstham Stone https://buildinglondon.blog/2022/02/25/32-stones-of-old-london-bridge-at-beaumont-quay/
And within a few year they realised they had made an big mistake as the stone started to flake and in the early 1900s a decisions was taken to re-face the whole building and this was done in 1906 and with Ancaster Stone: “In 1906 the exterior was refaced with Ancaster stone at a cost of £4,000.”  
Except it wasn’t all done! The back, unseen, was left. Maybe they ran out of money! And also, maybe they should have known that the stone they choose was a bad choice!
Martin Hatton in his MA “The Exploitation, Distribution and Use in Buildings of Reigate Stone” published by the Croydon Caving Society notes “In a report on the condition of the fabric of Westminster Abbey in 1713 Sir Christopher Wren stated that “that which is to be most lamented, is the unhappy Choice of the Material, the Stone is decayed four Inches deep, and falls of perpetually in great Scales” (Wren Society,1934,xi,p17). The ‘unhappy choice’ was “Rygate-stone in Surrey… used … for the Ashler of the whole Fabrick, which is now disfigured in the highest Degree” … The root cause of the poor weathering according to Wren was that “this Stone takes in Water, which being frozen, scales off, whereas good Stone gathers a Crust, and defends itself”.  
Hatton continues “To be fair, some of the stone Wren was looking at had been in place for almost 500 years. However it is also clear that the limitations of Reigate Stone must have been known from an early date: there is, for example, in the orders for rebuilding Eton College Chapel in c.1453, the requirement “that neyther in the seid growndes ne walles schall in any wise be occupied Chalke Bryke ne Reygate stone otherwise y called Mestham stone but oonly of the stuffe before rehersed” … Actions could be taken to mitigate the disadvantages of Reigate Stone. One of these was to harden it by protecting it from the rain, frost and sun when first exposing it to the air … Wren himself was aware of this and his contract for the supply of Reigate Stone for St. Paul’s required the quarrymaster to “house & keep the same [i.e. the Reigate Stone] dry & fitt for good work” … Furthermore, on at least three occasions – the winters of 1681,1683 and 1684 – he had his carpenters “making Sheds to cover the Rygat stones from the weather” … He does not appear to have treated any other type of stone with this level of care. Another mitigating action was to use Reigate Stone selectively; clearly it was sensible to use it in positions where it was protected from the depredations of the weather. The most obvious such position is inside buildings and there are many examples of this. However, there were also other positions where the cover was deemed adequate: hence in Canterbury Cathedral nave “Reigate [Stone] is seen most commonly for occasional large blocks on the inside of the aisle walls, as well as for the external ashlar work on the outside of the north aisle wall (where it is under cover in the cloister). Very worn Reigate stone work can also be seen outside the north door into the west end of the north aisle … Until the mid seventeenth century this stonework was under cover in a lobby area …” 
And even closer to Clapton than St Pauls or Eton the Clapton church could have looked at The Tower of London. Michael Michette another Reigate Stone expert who has studied in detail the significant decay of Reigate Stone in the Tower of London states “Reigate Stone was used as a freestone across south-east England from the eleventh until the sixteenth century, contributing signiﬁcantly to the re-emergence of masonry architecture in Britain during this period… Freestones are stones used for ashlar and ornamental masonry. Suitable lithologies can be cut freely in any direction and worked easily with a chisel; they tend to be ﬁne-grained, soft and homogeneous. Whilst this makes them easy to sculpt, it can also make them prone to rapid decay”. 
Francis G Dimes in Building Conservation of Building and Decorative Stone notes of the use of Reigate Stone in London in the medieval period as “Despite its poor weathering properties it was extensively used in London because it was quarried nearby” 
And Elsden and Howe in their seminal ‘Stones of London’ state “This stone is, like the above, soft when first quarried but hardens on drying; but it has not proved a durable material in London, when it was extensively used in early times…. Mouldering remain of this stone can still be examined in parts of the Tower of London and Westminster Abbey, as well as St Bartholomew’s the Great in Smithfield”
So for whatever reason, a bad batch of stone, a failure to look after it or just that Reigate stone seems to decay anyway the decision was taken to reface the chapel and they choose Ancaster stone. It would not have been as white as the Reigate and chose not to use another white limestone like Portland, but it’s clearly an attractive stone and has lasted extremely well. Over 110 years later there are only 1 or 2 of the 1000s of blocks with any flaking, while at the back, the small section that was left and rendered over, the render has collapsed off, probably pulling stone of the stone with it and the Reigate Stone continues to flake, though, again, tbf it is now 150 years old.
Building London will have future blog posts on the Clapton Round Chapel, particularly the incredible interior ironwork.
The Harries connection
The Building London blog is the work of a certain Glyn Harries and it would be remiss to note that it was another Harries (a name that is pretty rare and identified pretty rightly with Carmarthenshire / Sir Gaerfyrddin / Sir Gar ) who had the chapel refaced in 1906! The “Reverend Henry Harries…came to Clapton Park from Stockport in September, 1893. In his acceptance he had said “I was much interested in your Mission Stations (The Grove and Glyn Road) and to note your thought and care for the poor and needy in your neighbourhood. It will be my endeavour to foster these valuable agencies.” 
And big thanks to Phillip Lloyd at The Round Chapel for access and interest etc! Muc appreciated!
The Round Chapel is in Clapton, on Lower Clapton Road in between Glenarm Road and Powerscroft Road.
Easy access via public transport, buses stop outside and it’s not far from Hackney Central or Hackney Downs train stations on the London Overground or Liverpool Street lines. You can see the building fairly easily from the pavement and gates are often open. The old original Reigate Stone can be seen from Glenarm Road but not close up.
References  http://glebestone.com/stpancras.html
 Volume 10, Hackney, ed. T F T Baker (London, 1995), pp. 130-144. British History Online
 The Stones of London: a Descriptive Guide to the Principal Stones Used in London, With a Brief Non-Technical Account of Their Characteristic Features, 1923 James Vincent Elsden & John Allen Howe